I. Field of the Invention
The present invention relates generally to steerable drilling systems and more particularly to steerable drilling systems having on-demand directional control subassemblies for controlling bit trajectories in response to changing downhole conditions.
II. Description of the Prior Art
A steerable drilling system conventionally comprises a bit, a downhole motor, measurement while drilling components and stabilization. The system is designed to control bit trajectory without the need for tripping in both directional and straight hole applications. Steerable systems should provide the ability to: build, turn, hold or drop angle in a predictable manner; drill a smooth well path, minimizing dogleg severity; monitor wellpath with continuous directional surveys; allow surface control of bit trajectory in response to changing downhole conditions; optimize drilling parameters for fastest rate of penetration; and remain downhole drilling, requiring trips only to change bits or set casing.
To enable steering of the bit, steerable systems are designed for two modes: orienting and rotating. In the orienting mode, the drillstring is not rotated while the bit is turned by the downhole motor. The assembly is designed to impart a side load on the bit through either offset stabilizers or bends in the assembly. Sideloading causes the bit to deviate the wellpath. In the rotary mode, the drillstring is rotated in addition to bit rotation by the motor. The intentional bit sideloading rotates with the drillstring, thus negating its deviating effect.
There are three general categories of commercially available steerable systems: those systems which use eccentric stabilizers with downhole motors, described in U.S. Pat. No. 4,465,147; those systems which use adjustable bent subs above the motors; and motor housings with one or two bends, described in U.S. Pat. Nos. 4,667,751 and 4,932,482.
An offset stabilizer on the motor can be used on turbine or positive displacement motors, the greater the offset, the larger the rate of hole curvature. However, the amount of offset allowable is limited, which in turn limits the achievable hole curvatures (system response). This system type is sensitive to hole enlargement which acts to reduce the stabilizing effect.
A downhole-adjustable bent sub above the motor can be used as a steerable system. In the orienting mode, the system is essentially the same as a conventional directional drilling bottom hole assembly except for the ability to alter the bend angle downhole. Bend angles in the range of 1 to 3 degrees are required, which generate large bit offset and increased sensitivity to drilling parameters. Stabilization on the motor severely limits hole curvature in the orienting mode. In the rotary mode, the bend angle can be adjusted to zero, eliminating intentional bit sideloading. The system tends to drop angle without near bit stabilization in the rotary mode.
The majority of steerable system runs are with bend housing positive displacement motors (1 or 2 bends) and stabilization on the motor.
Heretofore, none of the above-mentioned steerable systems had the ability to correctly control bit trajectory in response to changing downhole conditions.